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On many old European maps, the river is named Menam or Mae Nam (Thai: แม่น้ำ), Thai for "river". James McCarthy, F.R.G.S., who served as Director-General of the Siamese Government Surveys prior to establishment of the Royal Survey Department, wrote in his account:

Me Nam is a generic term, Me signifying "mother" and Nam "water," and the epithet Chao P'ia signifies that it is the chief river in the kingdom of Siam.[2]

The rough co-ordinates of the river are 13 N, 100 E. This area has a wet monsoon climate, with over 1,400 millimetres (55 in) of rainfall per year, and temperatures range from 24 to 33 °C (75 to 91 °F) in Bangkok.

The lower Chao Phraya has undergone several modifications through man-made efforts during the Ayutthaya period. Several shortcut canals were constructed to bypass large meanders in the river, shortening the trip from the capital city to the sea. The course of the river has since changed to follow many of these canals.

The major bridges that cross the Chao Phraya are in the province of Bangkok: the Rama VI rail-road bridge; Phra Pin-klao near the Grand Palace; Rama VIII, a single tower asymmetrical cable-stayed bridge; Rama IX, a semi-symmetric cable-stayed bridge; and Mega Bridge, part of the Industrial Ring Road.

In Bangkok, the Chao Phraya is a major transportation artery for a vast network of river buses, cross-river ferries and water taxis, also known as longtails. More than 15 boat lines operate on the rivers and canals of the city, including commuter lines.

The principal tributaries of the Chao Phraya River are the Pa Sak River, the Sakae Krang River, the Nan River (along with its principal confluent the Yom River), the Ping River (with its principal confluent the Wang River), and the Tha Chin River.[7][8][9] Each of these tributaries (and the Chao Phraya itself) is further tributed by additional minor tributaries often referred to as khwae. All of the tributaries, including the lesser khwae, form an extensive tree-like pattern, with branches flowing through nearly every province in central and northern Thailand.[7] None of the tributaries of the Chao Phraya extend beyond the nation's borders.[10] The Nan and the Yom River flow nearly parallel from Phitsanulok to Chumsaeng in the north of Nakhon Sawan province. The Wang River enters the Ping River near Sam Ngao district in Tak province.

The expanse of the Chao Phraya River and its tributaries, i.e. the Chao Phraya river system, together with the land upon which falling rain drains into these bodies of water, form the Chao Phraya watershed.[11] The Chao Phraya watershed is the largest watershed in Thailand, covering approximately 35% of the nation's land, and draining an area of 157,924 square kilometres (60,975 sq mi).[12] The watershed is divided into the following basins:

Tha Chin Basin (the basin of the Chao Phraya's most significant distributaries)

Finally the Chao Phraya Basin itself is defined as the portion of the Chao Phraya Watershed drained by the Chao Phraya River itself, and not by its major tributaries or distributaries. As such, the Chao Phraya Basin drains 20,126 square kilometres (7,771 sq mi) of land.[12]

Meanwhile to the west the central plain of Thailand is drained by the Mae Klong and the east by the Bang Pakong Rivers which are not part of the Chao Praya system.

The landscape of the river basins is a very wide, flat, well-watered plain continuously refreshed with soil and sediment brought down by the rivers. The Lower Central plain from the delta north to Ang Thong Province is a flat, low area with an average of 2 m above sea level. Further north and into the plains of the Ping and the Nan the elevation is over 20 m. Then the mountains that are the natural boundary of the Chao Praya watershed form a divide, which has, to some degree, historically isolated Thailand from other Southeast Asian civilisations.

In fact in northern Thailand the divide roughly corresponds to a long section of the political border of the country today. Southern portions of the divide's boundary correspond less to the nation's political border, because isolation in this area was prevented by the ease of transportation along the lowlands surrounding the Gulf of Thailand, allowing a unified Thai civilisation to extend beyond the watershed without issue. The slightly higher northern plains have been farmed for centuries and saw a major change from the 13th century onwards during the Sukhothai Kingdom in the 13th and 14th centuries and the Ayutthaya Kingdom that succeeded it when rice-growing intensified with the introduction of floating rice, a much faster-growing strain of rice from Bengal. The southern swamps meanwhile changed radically from the 18th century when King Buddha Yodfa Chulaloke moved the capital of Siam to Bangkok, and a process of canalisation and cultivation began, especially as Thailand began to export rice from 1855 onwards.

The Tha Chin River is the major distributary of the Chao Phraya River. The expanse of the Chao Phraya and Tha Chin Rivers and their distributaries, starting at the point at which the distributaries diverge, together with the land amid the triangle formed by the outermost and innermost distributary, form the Chao Phraya Delta. The many distributaries of the Chao Phraya delta are interconnected by canals that serve both for irrigation and for transportation.

The original swamp forests have almost entirely been removed as the plain has been converted to rice paddies, other agriculture, and urban areas like the massive Bangkok, and much of the wildlife that once inhabited these plains has disappeared including a large number of fish in the river systems, birds such as vultures, the Oriental Darter (Anhinga melanogaster), White-eyed River Martin (Pseudochelidon sirintarae) and the Sarus Crane (Grus antigone)[14] and animals such as tigers, Asian elephants, Javan Rhinoceroses and the much-hunted Schomburgk's Deer.[15] Today we can only estimate a description of the original habitat and wildlife by comparing with that of neighbouring countries and it is believed that it would have consisted of freshwater swamps inland and salty mangroves on the coast and the river estuaries. The swamp would have been covered in Phragmites marsh grasses and today there is a small area of this remaining in Khao Sam Roi Yot National Park, a relic of the original landscape.

As so much has been cleared or altered the potential for creating large protected areas to preserve original habitat no longer exists. However much wildlife does remain in the rice fields and steps may be taken to preserve these as urban and industrial development on the plains is ongoing and the Industrial Estate Authority of Thailand has very little control or planning over this. Particular threats come from the conversion of rice paddies to large-scale production of prawns by pumping in seawater, and the use of pesticides to eliminate the introduced snail Pomacea canaliculata, which damages rice plants.

There are for example populations of threatened birds, including colonies of breeding water birds such as the world's largest populations of the near-threatened Asian Openbill (Anastomus oscitans), and other birds such as the wintering Black Kite (Milvus migrans). Endemic mammals that remain are the Limestone Rat (Niviventer hinpoon), Neill's Long-tailed Giant Rat (Leopoldamys neilli) and the near-endemic Thailand Roundleaf Bat (Hipposideros halophyllus).

The giant barb is one of worlds largest freshwater fish weighing up to 300 kg (660 lb),[17] but the natural population has been extirpated from Chao Phraya.[18]

The Chao Phraya basin is home to around 280 species of fish, including about 30 endemics.[19] By far the most diverse family is Cyprinidae with 108 species.[19] The mainstream of the Chao Phraya River has about 190 native fish species.[16] In general, the aquatic fauna of Chao Phraya and Mae Klong show clear similarities, and they are sometimes combined in a single ecoregion with 328 fish species.[16] Despite their similarities, there are also differences between the aquatic fauna of Chao Phraya and Mae Klong; the latter (but not the former) is home to a few taxa otherwise only known from major Burmese rivers: Irrawaddy, Salween and Tenasserim.[16] The aquatic fauna in Chao Phraya–Mae Klong also show clear similarities with that of the middle Mekong (the lower Mekong fauna more closely resembles that of the eastern Malay Peninsula).[16] It is believed that the upper Mekong was connected to Chao Phraya (rather than present-day lower Mekong) until the Quaternary, which explains the similarities in their river faunas. This included the Nan River basin, a tributary of the Chao Phraya, which is home to a number of taxa (for example, Ambastaia nigrolineata and Sectoria) otherwise only known from Mekong.[16] Of the fish species known from the Chao Phraya–Mae Klong, only about 50 are absent from the Mekong.[16]

There has been extensive habitat destruction (such as pollution, dams and drainage for irrigation) in the Chao Phraya basin and overfishing also presents a problem.[16][21][22][23] Within Mainland Southeast Asia, the only freshwater region with similar high levels of threats is the lower Mekong.[16] It has been estimated that only around 30 native fish species still are able to reproduce in the mainstream of the Chao Phraya River.[16]

The catfish Platytropius siamensis is endemic to Chao Phraya and Bang Pakong, but has not been recorded since the 1970s and is considered extinct.[24] Recent records of the near-endemic cyprinid Balantiocheilos ambusticauda are also lacking and it is possibly extinct.[16][25] Three of the largest freshwater fish in the world are native to the river, but these are all seriously threatened: The critically endangered giant barb (wild populations have been extirpated from Chao Phraya, but remain elsewhere),[18] critically endangered giant pangasius,[26] and endangered giant freshwater stingray.[27] The critically endangered red-tailed black shark, a small colourful cyprinid that is endemic to Chao Phraya, is commonly seen in the aquarium trade where bred in large numbers, but the only remaining wild population is at a single location that covers less than 10 km2 (4 sq mi).[20] The endangered dwarf loach, another species bred in large numbers for the aquarium trade, has been extirpated from most of its range in Chao Phraya.[28] The critically endangered Siamese tigerfish has been entirely exirpated from Chao Phraya and Mae Klong, but small populations remain in the Mekong basin.[29]

The basa fish from Chao Phraya and Mekong is an important food fish, and it is also farmed.[30]

^Tamara Loos (1 December 2002). "Introduction to Five Years in Siam". 1994 reprint. Pine Tree Web. Retrieved 1 March 2011. At the time of writing the "Introduction" to the 1994 reprint of Five Years in Siam, she was a PhD candidate in the Department of History at Cornell University.

^Molle, F. (2005). Elements for a political ecology of river basins development: The case of the Chao Phraya river basin, Thailand. Paper presented to the 4th Conference of the International Water History Association, December 2005, Paris.